To perform long-distance large-capacity transmission with an optical fiber, realization of high-density signal multiplexing, measures against signal power-to-noise power ratio deterioration due to optical amplitude, and measures against waveform distortion due to a fiber non-linear optical effect are problems.
As a technique for realizing large-capacity transmission, there is a method of increasing a transmission capacity per optical fiber by changing a modulation scheme in use to a modulation scheme with a large modulation multi-level number. There is also a method of increasing a transmission capacity by carrying different kinds of information on a plurality of optical carrier waves or optical sub-carrier waves and performing high-density wavelength multiplexing.
As a modulation scheme used in an optical transmission system, on off keying (OOK) for allocating binary signals to presence and absence of light and transmitting one bit per one symbol has been used. However, it is possible to increase a transmission capacity by using quaternary phase-shift keying (QPSK) or hexadecimal quadrature amplitude modulation (QAM) to increase the number of signal points and increase the number of transmission bits per one symbol.
There is known a scheme for increasing the number of transmission bits per one symbol to a double by using a polarization multiplexing scheme. In the polarization multiplexing scheme, signals are independently allocated to orthogonal two polarization components (vertical polarization and horizontal polarization).
For demodulation of an OOK signal, a direct detection scheme for detecting presence or absence of an optical signal and identifying the optical signal on a reception side has been used. For demodulation of a differential binary phase-shift keying (DBPSK) signal, a differential QPSK (DQPSK) signal, and the like, a scheme for, after delaying an optical signal and causing the optical signal to interfere, directly detecting the optical signal, that is, an optical-delay detecting scheme has been used. Most of signals transmitted using the polarization multiplexing scheme are demodulated using a digital coherent scheme for compensating, with digital signal processing, an electric signal obtained by performing coherent detection for mixing a local oscillation light source and a reception signal and causing the local oscillation light source and the reception signal to interfere to detect the local oscillation light source and the reception signal at a reception end. In the digital coherent scheme, a polarization multiplexing QPSK scheme is widely used (see, for example, Non Patent Literatures 1 and 2).
When a transmission capacity of an optical transmission system is expanded by multi-valuing of a modulation scheme, that is, by changing a modulation scheme in use to a scheme with a large modulation multi-level number, there is a problem in that a transmission distance is sometimes limited according to the multi-valuing of the modulation scheme. As measures against this problem, it has been examined to optimize a format of a wavelength multiplexed signal according to a transmission path of an optical signal and a combination of transmission paths (e.g., Patent Literature 1).
In a system for transmitting a large-capacity signal in a long distance, in a state in which signal quality cannot be kept at a specified value or more, optical signal reproduction and relay for once terminating an optical signal and converting the optical signal into an electric signal, converting the electric signal into an optical signal again, and transmitting the optical signal is performed (e.g., Patent Literature 2).